Multiple-input multiple-output (MIMO) is a promising technology for future wireless communication systems. By exploiting the multi-dimensional wireless channel created by multiple transmit and receive antennas, MIMO systems significantly increase the channel capacity and link robustness of wireless communication, and have been widely adopted in many future wireless communication standards (e.g., WiMAX, 3GPP LTE, etc).
FIG. 1 illustrates a multi-user MIMO system 100 comprising a base station 110, its antennas 115, and a number of user equipments (UEs) 120a-120c each being equipped with multiple antennas 125.
In a Multi-User MIMO (MU-MIMO) system, the single base station 110 transmits to multiple UEs 120a-120c simultaneously over the same frequency band, thereby substantially increasing the sum data-rate and reducing the latency of mobile users compared to other multi-access (MAC) schemes such as conventional TDMA or FDMA. The gains achievable by the introduction of MU-MIMO in wireless systems are forecast to play a major role in the increase of spectral efficiency of future wireless networks.
MU-MIMO is one way to improve the capacity of a wireless communication system. In contrast to Single-User MIMO (SU-MIMO) where several modulated data streams are sent to/from one user, MU-MIMO works by transmitting (or receiving) one (or more) data stream(s) to several users. All data streams use the same physical resource (time, frequency or code resource, etc.).
The current version of the WCDMA specification, e.g. Rel. 7, supports MIMO operation with dual stream transmission. Hence, two modulated data streams can be transmitted in parallel to a user, thus providing a peak data rate of 28 Mbps. This is accomplished by re-using the high-speed physical downlink shared channel (HS-PDSCH) spreading codes for each transmitted data stream.
It is also possible to schedule two users in the same TTI (Transmission Time Interval) using the same set of spreading codes. Hence, MU-MIMO operation is supported. In this case, only one stream (or transport block) can be scheduled to each user.
This can be done by addressing each user with its own control channel and thereby one High Speed Shared Control CHannel (HS-SCCH) is transmitted to each user.
Since the spreading codes are reused between the transmitted data streams, a code reuse (CR) interference term will exist in every TTI where dual stream transmission is performed. This is regardless of SU- or MU-MIMO operation. In case of SU-MIMO operation, this will be known in the terminal since this information is conveyed in the control channel information.
If the two transmitted data streams are meant for two different users, i.e. MU-MIMO operation is used, this information is not known to the UE, since each of the shared control channels indicates that only a single stream is scheduled to each user.
To some extent, this can be compensated for by the scheduler. When scheduling two users in parallel, the data rate to each user can be decreased to retain a decent probability of correct detection. The problem is that the scheduler does not know how large a back-off is needed for each user.
Alternatively, a user has to read all control channels, i.e. also those meant for other users, and from that information deduce if another data stream is scheduled or not. This will, of course, increase the burden on the UE and contribute to a more complex system.
Furthermore, other users' control channels may not always be reliably decoded since the users' control channels are power controlled separately for each user.